What If Gone

Species → Barn Owl

Barn Owl

Tyto alba

7,000,000 mature individuals worldwide, median of a wide estimated range
Extinction risk Least Concern
AgriculturalBiological controlPredator
Barn Owl (Tyto alba)
Photo: Luc Viatour, Wikimedia Commons (CC BY-SA 3.0)

What if it disappeared?

Impact Moderate
In California

Where barn owl activity is highest, vineyards have 38 to 52% fewer rodents, and surviving rodents show 16 to 38% more vigilance behavior — proof that the owl acts both by killing prey and by frightening the rest into changing their feeding behavior, a dual effect rarely measured together.

Documented
Detail

Larson, K., Baldwin, R.A. & Johnson, M.D. (2026). Barn owls exert top-down effects on the abundance and behavior of rodent pests. Ecological Applications.

The study combines giving-up-density food trays — a classic measure of perceived predation risk — with monitoring of owl activity at nearby nest boxes, across six separate vineyards. The dual effect (direct consumption and non-consumptive fear effect) is what sets this study apart from simple counts of prey reported at the nest.

Per breeding season

Camera-monitoring of nest boxes installed in Napa Valley vineyards directly quantifies the scale of predation: an average of 616 rodents delivered per pair per season (ranging from 358 to 899 depending on the box), confirming at the scale of a single operation the mechanics behind the effect measured in the field.

Documented
Detail

Johnson, M.D. & St. George, D. (2020). Estimating the number of rodents removed by barn owls nesting in boxes on wine-grape vineyards. Proceedings of the Vertebrate Pest Conference, 11(29), 18216-18228.

Cameras installed inside the nest boxes allow prey brought to the chicks to be identified directly, rather than inferring consumption from regurgitated pellets — a more direct method that reduces bias in identifying which rodent species were consumed.

Not measured yet

The commercial pitch for owl boxes is that they let growers cut back on anticoagulant rodenticides — themselves responsible for documented secondary poisoning in foxes, coyotes, and other raptors — but no study has measured whether installing nest boxes actually reduces the volume of rodenticide applied across an entire wine-growing region.

Insufficient
Detail

Available studies measure the owl's effect on the rodents themselves, not the change in growers' practices once boxes are installed — a grower could easily keep the boxes and keep using rodenticides in parallel, which would cancel out the intended environmental benefit.

Documentation

Last verified: August 14, 2026